The present invention relates to a method for precisely and positively measuring the separate thicknesses of a base metal and a clad metal layer thereon, the total thickness thereof with an electromagnetic crack test, and an apparatus for implementing the method.
So-called clad steel is broadly used in various industrial fields because it is very economical and also durable. Clad steel is so formed with a material such as stainless steel, titanium, aluminum, copper or an alloy thereof, which is different from the base metal, metallurgically overlaid on one side or both sides of the base metal such as carbon steel or low alloy steel by hot rolling, explosive adhesion or welding.
Such clad steel is particularly well adapted for use in a corrosive environment. However, in such applications, it is particularly important to maintain a proper thickness of the cladding material which forms the corrosion resistant layer.
Various methods for measuring the thickness of the cladding material or layer have been proposed. In a first such method, the thickness of the layer is mechanically measured with a measurement tool after etching away a peripheral portion. In a second prior art method, the overall thickness including that of the base metal is measured using a supersonic thickness gauge and the thickness of the cladding layer is calculated. In a third method, the thickness of the cladding layer is measured using an electromagnetic minute thickness detector to measure the variation of magnetic permeability caused by the cladding layer.
The above described conventional methods have the following drawbacks. The first mechanical method is only applicable for measuring a marginal portion of the clad steel. With this method, it is impossible to measure the thickness over the entire clad steel surface particularly at locally changed portions which may be formed due to forming compression. The second method in which a supersonic thickness gauge is employed may be used for measuring the thickness over the entire clad steel surface but it is impossible to use it to measure changes of the thickness due to variations in the forming rates of the cladding layer and the base metal. The third method wherein a electromagnetic minute thickness detector is used is applicable only to measurement of the cladding layer and its measuring precision is very low. In addition, the method is not applicable to the measurement of a cladding layer having magnetic properties.